lol no, it's alpine based. basing something on Manjaro would be pretty dumb anyways, might as well go straight off Arch (especially dumb since neither have official ARM support which phones need)

The biggest upside is that Alpine is small. The base installation is about 5 MB! Thanks to that, our development/installation tool pmbootstrap is able to abstract everything in chroots and therefore keep the development environment consistent, no matter which Linux distribution your host runs on. And if you messed up (or we have a bug), you can simply run pmbootstrap zap and the chroot will be set up again in seconds.

Another benefit of the tininess of Alpine - many older devices don't have much storage space to spare, so small system images can be anything ranging from useful to required.

https://postmarketos.org/faq/